Discussion on the two forms of heat treatment

Post-weld heat treatment generally involves either single high-temperature tempering or normalizing followed by high-temperature tempering. For gas welding joints, normalizing followed by high-temperature tempering heat treatment is applied. This is because the grains in the welds and heat-affected zones of gas welding are coarse, and it is necessary to refine these grains; therefore, normalizing treatment is employed. However, single normalizing cannot eliminate residual stresses; therefore, high-temperature tempering is required to remove them. Single medium-temperature tempering is only applicable to the assembly and welding of large ordinary low-carbon steel vessels assembled on-site, with the aim of partially eliminating residual stresses and dehydrogenating the material. In the vast majority of cases, a single high-temperature tempering is chosen. The heating and cooling during heat treatment should not be too rapid, in order to ensure uniformity across the inner and outer walls. When austenitic stainless steel is melted, the addition of titanium or niobium in amounts several times greater than the carbon content allows the formation of titanium or niobium carbides prior to the formation of Cr23C6; these carbides are almost insoluble in austenite. When the weld is cooled from high temperatures, even when passing through the sensitization temperature range (850–450°C) at which Cr23C6 tends to precipitate, large amounts of Cr23C6 do not precipitate along the grain boundaries, thereby **improving the resistance to intergranular corrosion. To achieve maximum stability in steel, stabilization treatment is also necessary; this involves heating the component to 900°C to ensure that Cr23C6 dissolves fully into the austenite, while at the same time allowing titanium and niobium to form highly stable titanium carbide and niobium carbide. It is then cooled in air, and no Cr23C6 precipitates at the grain boundaries even when passing through the sensitization temperature. Stabilized austenitic stainless steel **reduces the likelihood of intergranular corrosion. Key point: Post-weld heat treatment is used for carbon steel, while stabilization is used for stainless steel